1. Field of the Invention
The present invention relates to an impedance measurement method, and relates to an impedance measurement device utilizing that impedance measurement method.
2. Description of Related Art
Due to the speed increase in digital circuits, the I/O circuit's switching speed has been becoming faster than ever before, which causes a larger transient current within the circuit. Furthermore, when the multiple transistors in the I/O circuit switch simultaneously, a noticeably large SSN (Simultaneous Switching Noise) is generated that exerts adverse effects on the circuit.
In order to prevent the noise caused by the SSO (Simultaneous Switching Output), the digital circuits, not only the silicon circuit but also package and printed circuit board should be designed to have an appropriate operating margin. Therefore, a highly accurate equivalent circuit must be prepared for each component within the digital circuit, which requires a highly accurate impedance value of the each circuit component.
FIG. 1 explains the impedance measurement method disclosed in JP-A No. 2006-258667. In this measurement method, the S-parameter between the multiple terminals is measured with the multiple RF (Radio Frequency) probes that are connected to a network analyzer. The S-parameter acquired with a network analyzer is then converted into a Z-parameter in order to obtain the impedance of the printed circuit board. In detail, the S-parameter of the power supply terminal 113 on the surface of the circuit wiring board is measured with the ground terminal 308 as the reference, by using RF (Radio Frequency) probe 303 and an RF probe 302 that are connected to a network analyzer. In this case, the necessary impedance to be obtained is the impedance between the ball grid 111 and the power supply terminal 113. However, because of the support board 220, a direct contact with the RF probe 302 to the ball grids 111 is practically impossible.
A method conducting this kind of impedance measurement is described in the U.S. Pat. No. 6,737,875 which discloses a method for measuring the impedance between the two terminals without the need for removing the circuit elements connected to the each terminal.
The present inventor has recognized that: if a direct contact with the RF probe to the measurement terminal is impossible, a support board 220 is utilized that makes the measured value to include the impedance of the support board 220 it self. In general, the measured value includes the unwanted parasitic impedance of the measurement system if RF probe cannot be directly contacted to the terminal. Moreover, in this case, because the power supply impedance and ground impedance in the printed circuit wiring board are not separately measured, the obtained impedance is inaccurate, especially in high-frequency region.